It is known to protect subscriber line interface circuits against overvoltages, primarily against overvoltages in the form of lightening-induced overvoltage pulses caused, with the aid of a so-called primary protector which may, for instance, consist in a spark gap which functions to reduce the overvoltages on the subscriber line to a voltage of about one kilovolt. A so-called secondary overvoltage protector is fitted to the subscriber line, to deal with these residual pulses of about one kilovolt. The secondary overvoltage protector may have the form of a known circuit marketed by General Semiconductor under the trademark TRANSZORB, or a circuit marketed by RCA under the trademark SURGECTOR.
Current standards require the secondary overvoltage protector to be earthed to the apparatus stand with a separate wire. However, this can create problems that are manifested in the form of residual pulses in the order of magnitude of some hundred volts across a subscriber line interface circuit connected to the subscriber line. Modern line interface boards, in which the line interface circuit is a monolithic integrated circuit, are very often broken-down by lightening, due to residual pulses from the secondary overvoltage protector appearing across the two input wires of the circuit board.
U.S. Pat. No. 4,456,940 teaches an overvoltage protector which includes, among other things, two diodes whose anodes are each connected to a respective input terminal of a subscriber line interface circuit and whose cathodes are connected to earth, and further includes two thyristors whose cathodes are each connected to a respective input terminal on the subscriber line interface circuit and whose anodes are connected to earth. The thyristor gates are connected to the supply voltage of the subscriber line interface circuit. This protective circuit protects the subscriber line interface circuit against overvoltages. However, the thyristors are relatively space-consuming and a reduction in their number would be a significant advantage. It is sometimes necessary to match the thyristors, so that they will ignite or fire simultaneously, i.e. for the same voltage across the cathodes with equal voltage across the gate. However, the document does not describe several cascade-connected overvoltage protectors.